Switch to external genmanifest tool

Until now, we were using a Go program included in this repo to generate
the manifest. This program is being used in other projects, so it got
moved to the gomodules/webext repo.

This change removes the local Go files and switches to the external
genmanifest tool.

Change-Id: Id6168282122ae53ecb9e20a296d2f223e66b9f25
diff --git a/roles/build-extension/tasks/main.yaml b/roles/build-extension/tasks/main.yaml
index 7759c47..6ca0225 100644
--- a/roles/build-extension/tasks/main.yaml
+++ b/roles/build-extension/tasks/main.yaml
@@ -3,4 +3,4 @@
     chdir: "{{ zuul.project.src_dir }}"
     target: "{{ make_target }}"
   environment:
-    PATH: /usr/local/go/bin:{{ ansible_env.PATH }}
+    PATH: "{{ ansible_env.HOME }}/go/bin:/usr/local/go/bin:{{ ansible_env.PATH }}"